The invention relates to the use of citric acid tri-alkylamides to reduce the surface tension in water-based systems.
The ability to reduce the surface tension of water is of great importance in waterborne coatings, inks, adhesives, fountain solutions, agricultural formulations and cleaning compositions for electronics processes such as semiconductor manufacture because decreased surface tension translates to enhanced substrate wetting in actual formulations. Surface tension reduction in water-based systems is generally achieved through the addition of surfactants. Performance attributes resulting from the addition of surfactants include enhanced surface coverage, fewer defects, and a more uniform distribution. Equilibrium surface tension performance is important when a system is at rest. However, the ability to reduce surface tension under dynamic conditions is of great importance in applications where high surface creation rates are utilized. Such applications include the spraying, rolling and brushing of coatings or agricultural formulations, or high speed gravure or ink-jet printing. Dynamic surface tension is a fundamental quantity which provides a measure of the ability of a surfactant to reduce surface tension and provide wetting under high speed application conditions.
Traditional nonionic surfactants such as alkylphenol or alcohol ethoxylates and ethylene oxide (EO)/propylene oxide (PO) copolymers have excellent equilibrium surface tension performance but are generally characterized as having poor dynamic surface tension reduction. In contrast, anionic surfactants such as sodium dialkyl sulfosuccinates can provide good dynamic results, but are very foamy and impart water sensitivity to the finished coating.
In addition to the development of high-performance surfactants, there is considerable interest in the industry in surfactants with improved environmental characteristics. Environmental concerns have led to an increased use of environmentally compatible surfactants as alternatives have become available. In addition, the use of less favorable products, such as alkylphenol ethoxylate (APE) surfactants, has declined. This is, in part, due to the poor environmental characteristics of APE surfactants, such as incomplete biodegradation and a suspicion that they may function as endocrine mimics.
The demand for high-performance, eco-friendly surfactants has stimulated efforts in new surfactant development. From this work a new family of surfactants, referred to as alkyl polyglycoside (APG) surfactants, has emerged as a readily biodegradable, environmentally-friendly alternative to conventional surfactants. These materials, however, can be foamy and thus, are not suitable for a variety of coating, ink, adhesive and agricultural applications where the generation of foam is undesirable. Thus, not only is it desirable to obtain surfactants which exhibit excellent surface tension reducing capabilities, but it is also desirable that these surfactants exhibit low foam under dynamic application conditions and are environmentally friendly.
There is a need for surfactants which exhibit good equilibrium and dynamic surface tension properties, are low-foaming, and would be widely accepted in the waterborne coating, ink, adhesive, fountain solution, agricultural formulation and electronics manufacturing industries. Moreover, since there is substantial interest in the development of environmentally friendly surfactants, an essential attribute would be that these new surfactants not only possess the aforementioned desired performance attributes but also are derived from natural, renewable resources.
The importance of reducing equilibrium and dynamic surface tension in applications such as coatings, inks, adhesives, fountain solutions, agricultural formulations and electronics cleaning compositions, e.g., aqueous developer solutions for making semiconductor devices, is well-appreciated in the art.
Low dynamic surface tension is of great importance in the application of waterborne coatings. In an article, Schwartz, J. xe2x80x9cThe Importance of Low Dynamic Surface Tension in Waterborne Coatingsxe2x80x9d, Journal of Coatings Technology, September 1992, there is a discussion of surface tension properties in waterborne coatings and a discussion of dynamic surface tension in such coatings. Equilibrium and dynamic surface tension were evaluated for several surface active agents. It is pointed out that low dynamic surface tension is an important factor in achieving superior film formation in waterborne coatings. Dynamic coating application methods require surfactants with low dynamic surface tensions in order to prevent defects such as retraction, craters and foam.
Efficient application of agricultural products is also highly dependent on the dynamic surface tension properties of the formulation. In an article, Wirth, W.; Storp, S.; Jacobsen, W. xe2x80x9cMechanisms Controlling Leaf Retention of Agricultural Spray Solutionsxe2x80x9d; Pestic. Sci. 1991, 33, 411-420, the relationship between the dynamic surface tension of agricultural formulations and the ability of these formulations to be retained on a leaf was studied. These workers observed a good correlation between retention values and dynamic surface tension, with more effective retention of formulations exhibiting low dynamic surface tension.
Low dynamic surface tension is also important in high-speed printing as discussed in the article xe2x80x9cUsing Surfactants to Formulate VOC Compliant Waterbased Inksxe2x80x9d, Medina, S. W.; Sutovich, M. N. Am. Ink Maker 1994, 72 (2), 32-38. In this article, it is stated that equilibrium surface tensions (ESTs) are pertinent only to ink systems at rest. EST values, however, are not good indicators of performance in the dynamic, high speed printing environment under which the ink is used. Dynamic surface tension is a more appropriate property. This dynamic measurement is an indicator of the ability of the surfactant to migrate to a newly created ink/substrate interface to provide wetting during high speed printing.
Tetramethylammonium hydroxide (TMAH) is the chemical of choice in aqueous alkaline solutions for developing photoresists according to Microlithography, Science and Technology, edited by J. R. Sheats and B. W. Smith, Marcel Dekker, Inc., 1998, pp 551-553. Surfactants are added to the aqueous TMAH solutions to reduce development time and scumming and to improve surface wetting.
U.S. Pat. No. 5,562,762 discloses an aqueous jet ink of water, dissolved dyes and a tertiary amine having two polyethoxylate substituents and that low dynamic surface tension is important in ink jet printing.
Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) is found in nature and is a common metabolite for many plants and animals. It is classified as GRAS (Generally Recognized As Safe) by the U.S. Food and Drug Administration and is commonly used in foods, pharmaceuticals, detergents, cosmetics, cleaners and enhanced oil recovery. Citric acid is produced by the fermentation of sugars, such as corn starch or molasses.
Anionic surfactants based on citric acid amides are known.
JP 08302387A discloses the use of mono-amides prepared from citric acid and C10-C20 amines as surfactants in cosmetic formulations. These anionic surfactants are foamy and, thus, not suited for coating applications, such as the spraying, rolling and brushing of coatings or agricultural formulations, or high speed gravure or ink-jet printing.
U.S. Pat. No. 3,946,074 discloses di- and tri-amides of aliphatic polycarboxylic acids, including citric acid as growth regulators when applied to plants as an organic or aqueous solution or as an aqueous emulsion. The citramide, N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-Nxe2x80x3-1-methyldecyl citramide, was synthesized in Example 1. JP06100833 discloses aqueous ink compositions containing water soluble dyes or water dispersible pigments, a water soluble organic solvent and a tri-carboxylic acid amide.
GB 944515 discloses citric acid amides containing as part of the amide residue at least one residue of an alkylene diamine carrying on one nitrogen atom a C10-C22 aliphatic hydrocarbon residue. Citric acid amides of this type were shown to be active as bactericides and fungicides in aqueous compositions.
JP 09286768 discloses tri-amides of citric acid containing C8-C22 alkyl or alkylene groups, e.g., trioctyl- and tridodecylamides of citric acid, as lubricants, releases and dispersants for the processing of thermoplastic and thermosetting resins and synthetic rubber. In all examples, the citramide additive was added to non-aqueous compositions.
U.S. Pat. No. 5,776,494 discloses the use of alkylamides of di-and/or tribasic carboxylic acids, e.g., 2-hydroxy-1,2,3-propanetributylamide, as gelling agents in pharmaceutical compositions in the form of a gel or gel stick. The compositions are prepared using an anhydrous liquid carrier which contains  less than 5% water.
The invention provides water-based compositions which contain an organic or inorganic compound, particularly aqueous organic protective or decorative coating, ink, adhesive, fountain solution, agricultural and electronics cleaning compositions, having reduced equilibrium and dynamic surface tension by incorporation of an effective amount of a tri-alkylamide of citric acid (xe2x80x9ccitramidexe2x80x9d) of the formula: 
where R1, R2 and R3 are independently C1 to C18 alkyl groups. It is desirable that an aqueous solution of the citramide demonstrates a dynamic surface tension of less than 50 dynes/cm at a concentration of xe2x89xa65 wt % in water at 25xc2x0 C. and 20 bubble/second according to the maximum bubble pressure method. The maximum bubble pressure method of measuring surface tension is described in Langmuir 1986, 2, 428-432, which is incorporated by reference.
By xe2x80x9cwater-basedxe2x80x9d, xe2x80x9caqueousxe2x80x9d or xe2x80x9caqueous mediumxe2x80x9d, we mean, for purposes of this invention, a solvent or liquid dispersing medium which comprises at least 90 wt %, preferably at least 95 wt %, water. Obviously, an all water medium is also included. In addition, it is desirable that the water-based compositions are essentially free of hydrocarbon solvent.
Also provided is a method for lowering the equilibrium and dynamic surface tension of such aqueous compositions by the incorporation of these citramide compounds.
Also provided is a method for applying a coating of a water-based inorganic or organic compound-containing composition to a surface to partially or fully coat the surface with the water-based composition and drying the composition to deposit a coating, the composition containing an effective amount of a citramide compound of the above structure for reducing the dynamic surface tension of the water-based composition.
There are significant advantages associate d with the use of these citramides in water-based organic coatings, inks, adhesives, fountain solutions, agricultural and electronics chemical compositions and these advantages include:
water-borne coatings, inks, adhesives, fountain solutions, agricultural and electronics chemical formulations which may be applied to a variety of substrates with excellent wetting of substrate surfaces;
a reduction in coating or printing defects such as orange peel and flow/leveling deficiencies;
coating and ink compositions capable of high speed application;
low-foam surfactants capable of reducing dynamic surface tension;
low-foam surfactants which have low odor;
water-borne compositions using a surfactant derived from natural, renewable resources, thus making such formulations environmentally favorable; and
an ability to formulate low surface tension aqueous electronics cleaning and processing solutions, including photoresist developer solutions, for the semiconductor manufacturing industry with good wetting and extremely low foam.
Because of their excellent surfactant properties and the ability to control foam, these materials are likely to find use in many applications in which reduction in dynamic and equilibrium surface tension and low foam are important. Applications in which low foam is important include various wet-processing textile operations, such as dyeing of fibers, fiber scouring, and kier boiling, where low-foaming properties would be particularly advantageous; they may also have applicability in soaps, water-based perfumes, shampoos, detergents, cleaners, cosmetics and food processing where their marked ability to lower surface tension while simultaneously producing substantially no foam would be highly desirable.
In addition, the demands of the semiconductor fabrication industry have led to the requirement for high performance surfactants and wetting agents for photoresist developer formulations. As line features shrink to smaller sizes and photoresist substrate materials become more aliphatic in nature (i.e., lower surface energy), aqueous developer solutions increasingly are being formulated with surface tension reducing agents.
An additional requirement for these developers, accentuated by the move toward larger wafer sizes, is that they exhibit low foam. This is particularly important when the so-called spray puddle techniques are used in applying the developer solution, wherein the developer is sprayed over increasingly larger areas. Even in cases where puddle or immersion techniques are used, microbubble entrainment during spreading of the solution over the photoresist surface can lead to defects. The materials according to the present invention give efficient reduction of surface tension of aqueous developer solutions and exceedingly low foam, even under extreme conditions. Other applications in the electronics industry using aqueous processing media would also benefit from good dynamic wetting and low foam.
This invention relates to the use of citramide compounds of the structure 
where R1, R2 and R3 are C1 to C18 alkyl groups, preferably C2 to C8 and most preferably C3 to C5, for the reduction of equilibrium and dynamic surface tension in water-based compositions which are essentially free of aliphatic and aromatic hydrocarbon solvents and contain an organic compound, particularly protective or decorative coating, ink, fountain solution, adhesive, agricultural and electronics processing compositions containing organic compounds such as polymeric resins, organic bases, herbicides, fungicides, insecticides or plant growth modifying agents. It is desirable that an aqueous solution of the citramide demonstrates a dynamic surface tension of less than 50 dynes/cm at a concentration of xe2x89xa65 wt % in water at 25xc2x0 C. and 20 bubble/second according to the maximum-bubble-pressure method.
In one aspect of the invention the citramides of the above formula display excellent ability to reduce equilibrium and dynamic surface tension while producing substantially no foam.
These materials may be prepared by the reaction of primary amines with citric acid or citric acid esters. The reaction is illustrated below: 
The amination reaction to form the amide may be performed using a variety of conditions well known in the organic chemical art as described in the Kirk-Othmer Encyclopedia of Chemical Technology, 4 th Ed., Vol. 2, p. 348-351. The preferred method involves the reaction of a citric acid ester with 3 or more equivalents of amine in a protic solvent.
All primary amines or mixtures of primary amines containing the requisite C1 to C18 alkyl substituents may be utilized for the preparation of the tri-alkylcitramides of this invention, with amines containing 2-8 carbons being preferred and those containing 3-5 carbons being especially preferred. It is preferred that the tri-alkylcitramides contain a total of 8-20 amide alkyl carbons and those containing a total of 9-15 amide alkyl carbons are especially preferred. Alkyl groups which are suitable should have sufficient carbon atoms to confer surface activity (i.e. an ability to reduce the surface tension of water) to the material but not enough carbon atoms to decrease the solubility to the extent that the ability of the material to reduce surface tension is insufficient for a particular application. In general, an increase in the carbon number increases the efficiency of the resulting tri-alkylcitramide (i.e. less surfactant is required to obtain a given decrease in surface tension), but it decreases the surfactant""s ability to reduce surface tension at high surface creation rates. The latter effect is a result of the fact that increased carbon number generally decreases the water solubility of the material, and consequently, diminishes the diffusive flux of surfactant to newly-created surfaces. Generally, in the practice of this invention, it is desirable to choose amide alkyl groups such that the resulting tri-alkylcitramides have a solubility limit in water of xe2x89xa610 wt % and preferably from 0. 1 to 0.5 wt %
The alkyl groups in the citramides may be the same or different and may be linear or branched. Examples of suitable alkyl groups are methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, n-pentyl, 2-pentyl, 3-pentyl, iso-pentyl, neopentyl, cyclopentyl, 2-methylbutyl, 3-methyl-2-butyl, n-hexyl, 2-hexyl, 3-hexyl, cyclohexyl, 2-ethylbutyl, 4-methyl-2-pentyl, n-heptyl, n-octyl, n-2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl and so on. Of these derivatives those which contain a total of 8 to 20 amide alkyl carbons are preferred and those containing 9 to 15 alkyl carbons especially preferred, with those containing 12 alkyl carbons being the most preferred, especially in the case where R1=R2=R3=butyl.
An amount of tri-alkylcitramide that is effective to reduce the equilibrium and/or dynamic surface tension of the water-based, organic compound-containing composition may range from 0.001 to 20 wt %, preferably 0.01 to 10 wt %, of the aqueous composition. Naturally, the most effective amount will depend on the particular application and the solubility of the citramide. The citramides are suitable for use in an aqueous composition comprising in water an inorganic compound which is, for example, a mineral ore or a pigment or an organic compound which is a pigment, a polymerizable monomer, such as addition, condensation and vinyl monomers, an oligomeric resin, a polymeric resin, a detergent, a cleaning agent, a dissolution agent such as trimethylammonium hydroxide (TMAH), a herbicide, a fungicide, an insecticide, or a plant growth modifying agent.
In the following water-based organic coating, ink, adhesive, fountain solution, agricultural and photoresist developer compositions containing a tri-alkylcitramide according to the invention, the other listed components of such compositions are those materials well known to the workers in the relevant art.
A typical water-based protective or decorative organic coating composition to which the tri-alkylcitramide surfactants of the invention may be added would comprise in an aqueous medium 30 to 80 wt % of a coating composition containing the following components:
A typical water-based ink composition to which the citramide surfactants of the invention may be added would comprise in an aqueous medium 20 to 60 wt % of an ink composition containing the following components:
A typical water-based agricultural composition to which the citramide surfactants of the invention may be added would comprise in an aqueous medium 0.1 to 80 wt % of an agricultural composition containing the following components:
A typical water-based fountain solution composition containing the citramide surfactants of the invention would comprise the following components:
A typical water-based adhesive composition to which the tri-alkylcitramide surfactants of the invention may be added would comprise in an aqueous medium 30 to 65 wt % of an adhesive composition containing the following components:
A typical water-based photoresist developer, or electronic cleaning, composition to which the tri-alkylcitramide surfactants of the invention may be added would comprise an aqueous medium containing the following components: